The present invention relates to a process and structure for producing etched openings in films. The etched openings include retrograde features. More particularly, the present invention is related to processes and structures used to create a retrograde opening, within a thin film having a characteristic which forms a gradient from the top to the bottom of the film, and to fill the opening with an electrodeposited film.
In the modem semiconductor device manufacturing industry, there exists a need to form openings having retrograde features within the thin films used in the formation of semiconductor integrated circuit devices. A retrograde opening extending down from a top surface of a thin film includes a bottom section having a greater lateral dimension than the top section of the opening. Relatively straight sidewalls give the cross section of the opening a tapered or trapezoidal appearance. Retrograde openings may be subsequently filled with materials which are used in various applications.
One such application may be to form a conductive material within the retrograde opening using chemical vapor deposition (CVD) techniques. A material formed within a retrograde opening using CVD techniques may additionally include air gaps along the sides of the conductive material and within the retrograde opening. If, for example, damascene processing techniques are used to form a wiring pattern of the conductive material, then the parasitic capacitance between adjacent conductive lines may be reduced because of the lowered dielectric constant of the air gap, as compared to that of the dielectric film within which the opening is formed.
Another application of a retrograde opening within a thin film is the subsequent electrodeposition of a conductive film to completely fill the retrograde opening. After the excess conductive material is removed from the top surface of the film, a wiring pattern is produced of the electrodeposited conductive material. If the dielectric film, in which the original retrograde opening was formed, is subsequently removed, a wiring pattern results. The wiring pattern includes wires having tapered cross-sections. In the modern semiconductor processing industry, many films which form desirable interconnect materials, such as platinum, are resistant to etching using conventional methods. It is therefore difficult to create a controllably tapered wiring pattern by forming a continuous platinum film on a substrate surface, then selectively removing portions of the film by etching. In addition, wet and plasma chemical etching techniques are generally isotropic in nature, etching equally in all directions, and cannot be used to produce retrograde features having controlled configurations.
Conventional reactive ion etch (RIE) processes are generally anisotropic processes which etch in one direction and form openings having a generally vertical profile. Conventional RIE processes commonly depend upon the formation of generally polymeric bi-products formed during the etch process. These bi-products adhere to sidewalls or other exposed etch features, thereby passivating the exposed surfaces and influencing the shape of the final, formed opening. Because of the passivation of the bi-products, openings formed by conventional RIE (dry) etching processes generally have an angle of 90xc2x0 or larger (i.e., not retrograde) between the bottom and sidewalls.
Attempts directed to forming a retrograde opening within thin films have generally included forming an etchable homogenous film, then etching the film using an etch process whereby the etch conditions are varied during the etch process to alter the etch rate or the degree of isotropy of the etch during the course of the etch process. RIE processes are typically used for such an application. RIE conditions such as gas flow, temperature, pressure, and RF power can typically be varied during the course of an etch process using automated equipment available in the modern semiconductor device manufacturing industry.
The non-conventional RIE processes commonly used in attempting to form retrograde openings within thin films are sensitive processes which are difficult to continuously control during the course of a dynamic etch process and also from run-to-run. As noted above, RIE processes commonly depend upon the formation of generally polymeric bi-products formed during the etch process, which passivate exposed surfaces by adhering to them. The formation and adhesion of these passivating etch bi-products are very sensitive to localized processing conditions and, therefore, are difficult to control.
Other attempts to form retrograde openings within thin films include forming a pattern above an initially deposited, relatively homogenous film; introducing impurities into discrete sections within the deposited film; then re-patterning and etching, using the different etch characteristics of the discrete, impurity sections of the film with respect to the bulk, as-deposited film. These processes require multiple patterning operations having tight alignment tolerances.
The present invention is directed to the shortcomings of conventional methods for forming retrograde openings within the thin films used in the formation of semiconductor devices. The present invention provides an etchable film having a characteristic which varies from the top to the bottom of the film. The etch rate of the film for a chosen etchant species is sensitive to this characteristic which forms a gradient within the film. In one embodiment, the film may be formed as a series of individually formed sub-layers.
The present invention provides a two-step process including a first anisotropic etch process, followed by an isotropic etch process using an etchant species that is selective to the characteristic which is varied from top to bottom of the film. The isotropic etch process is conducted after an anisotropic etch process is used to form openings within the film. The isotropic etch process converts the anisotropic openings to openings having retrograde characteristics. The present invention also forms a wiring pattern by providing conductive material within the retrograde openings by electrodeposition or other known deposition mechanisms.